1. Field of the Invention
The present invention pertains generally to methods and apparatus used in evaluation of cardiovascular health, and more particularly to the evaluation of cardiovascular health by techniques of arterial endothelial function measurement by vasorelaxation or vasoconstriction.
2. Description of the Relevant Art
Most measurements of arterial endothelial function are limited to researchers due to the cost of such measurements. Typically, such measurements have low reliability, and need further improvement before an average physician would recommend arterial endothelial function tests for the detection of symptoms indicative of extremely early stage atherosclerosis. Presently, arterial endothelial function tests are done by ultrasound imaging using the same devices already commercially available for internal organ imaging. Such devices are ill disposed to maintain constant skin contact and precisely image arteries.
Voss et al. U.S. Pat. No. 5,848,970 (the '970 patent), hereby incorporated by reference, discloses an apparatus for non-invasively monitoring a subject's blood pressure. In the '970 patent, a flexible diaphragm enclosing a fluid-filled chamber is compressed against the tissue overlying an artery with sufficient force to compress the artery. However, the invention described herein measures endothelial function by the calculation of arterial elasticity.
Voss et al., in U.S. Pat. No. 6,228,034 (the '034 patent), also hereby incorporated by reference, relates generally to apparatus and methods for monitoring a subject's arterial blood pressure and, more particularly, to such apparatus and methods that monitor arterial blood pressure non-invasively by applying a pressure sensor against tissue overlying an arterial blood vessel, to partially applanate or compress the vessel.
Sato et al. in U.S. Pat. No. 5,033,472, hereby incorporated by reference, provides a non-invasive method and apparatus for automatically analyzing the fluctuation and distribution of the propagation time of arterial pulse waves. Such method and apparatus are based upon a fact that the degree of fluctuation in the propagation time of the arterial pulse wave from the center of the circulatory system (i.e. heart) to the periphery of the circulatory system, can serve as a diagnostic index of vascular wall tensions or an indication of diseases through a statistical analysis of such propagation times. The invention described herein measures the pulse transit time in an artery from an artificially induced pulse to a sensor.
Biehl et al., in U.S. Pat. No. 6,200,270, attached hereto and incorporated by reference herein, provides a sensor for non-invasive and continuous determination of the duration of arterial pulse waves, but may not be successfully used to measure arterial endothelial function.
Similarly, Hood et al., in U.S. Pat. No. 5,680,870, attached hereto and incorporated by reference herein, provide an automated sphygmomanometer that measures static blood pressure, but not endothelial function.
Drzewiecki et al., U.S. Pat. No. 6,626,840, hereby incorporated by reference, describe an occlusive arm cuff plethysmograph to measure the flow of air, the pulse pressure waveform and the change in air volume in a blood vessel to measure arterial endothelial function. This is done by looking at pressure versus lumen area curves, which in turn provides information on the compliance and elasticity of the vessel. Drzewiecki et al. do not use pulse wave velocity and do not output a direct pulse wave velocity measurement. The method is dependent on uniform arterial dilation and calibration, limitations that the present invention does not have. Additionally, the present invention measures pulse wave velocity directly, and does not use an occlusive cuff plethysmograph.
Loukogeorgakis et al. in “Validation of a device to measure arterial pulse wave velocity by a photoplethysmographic (PPG) method” [Physiol. Meas. 23 (August 2002) 581-596] measure the pulse wave velocity from an echocardiograph (ECG) “R” wave by determining propagation velocity of the diameter wave of a vessel using an infra-red emitting diode probe and a phototransistor detector. They also compared Doppler ultrasound and the PPG method for measuring pulse wave delay (the time between the ECG “R” wave and the foot of the arterial pulse wave). In the present invention, an ultrasonic method of measuring pulse wave velocity is used. Additionally, a low-pressure artificial pulse is induced rather than the normally occurring arterial pulse. The present invention's low-pressure artificial pulse minimizes the non-linear response region of the elastic properties of the arterial wall, which occurs during somatic cardiac pulse propagation. The artificial pulse also has a more clearly defined rising edges than a somatic cardiac pulse, which greatly improves the timing accuracy and resolution for pulse transit times. Non-linear acoustic dispersive effects tend to blur these edges owing to dispersion of frequency components in the pulse spectrum.